This invention relates to electronic or ionic optical apparatus, which is understood as meaning any apparatus through which electrically charged particles pass, such as electrons, protons, ionized atoms or molecules or the like, and in which fields affecting the movement of the electrically charged particles prevail. Examples of such apparatus are any energy-dispersive or mass-dispersive spectrometers and monochromators for charged particles, which are formed, for example, of spherical condensers, spherical mirrors, cylindrical condensers, cylindrical mirrors, plate condensers and the like. Also to be included are electrostatic or magnetostatic lens systems.
In all these systems the suppression of background is problematical. The background is due mainly to some of the charged particles which pass through the system, and which, on account of excessive angular divergence or excessive departure from the required energy, impinge upon the inner walls of the system where they rebound or produce secondary particles.
Many different measures have been taken to suppress the background created in this manner. One measure consists in using masks to limit the beam of particles before it enters the system. This, however, only reduces the angular divergence, but not the velocity distribution, so that a certain reduction of the background is achieved but it entails considerable intensity losses. The reason for this is that the divergence angle must be restricted much more severely than necessary in order to mask out particles whose divergence angle is small, but whose velocity departs excessively from the desired velocity.
It has been proposed in German Offenlegungsschrift No. 2,639,033 to make the wall portions seen by the electrically charged particles from materials having a low dusting rate and a low desorption rate, and to provide in these wall areas a plurality of small, closely adjacent apertures. It has also been proposed by Froitzheim et al. in Rev. Sci. Instrum., Vol. 46, No. 10, pp. 1325-28, Oct. 1975, to provide analyzer plates with a sawtooth configuration. The use of anti-scatter baffles has been proposed by Elliott et al. in Nuclear Instruments and Methods, 59 (1968) pages 29-39. These proposals, however, result in a certain background suppression, but one which is disproportionate to the increased cost of the production of the wall portions.